1. Field of the Invention
The present invention relates to an ink-jet head for ejecting droplets of ink onto a print surface to make an image on the print surface, and a manufacturing method of the ink-jet head.
2. Description of Related Art
An ink-jet head is known in which an actuator unit is bonded to a passage unit. The passage unit includes therein pressure chambers each connected to a nozzle. The actuator unit includes therein active portions for changing the volumes of the respective pressure chambers. In the ink-jet head, in many cases, the actuator unit includes a piezoelectric ceramic sheet portions of which sandwiched by electrodes function as the respective active portions. When a portion of the polarized piezoelectric ceramic sheet sandwiched by electrodes receives, through the electrodes, an electric field along the polarization, the portion of the piezoelectric ceramic sheet is extended or contracted along the thickness of the sheet. Thereby, the volume of the pressure chamber corresponding to the active portion is changed to eject ink through the nozzle connected to the corresponding pressure chamber.
Such piezoelectric ceramic sheets are made through baking process, and thus green sheets to be baked are prepared with taking account of shrinkage upon baking. However, the shrinkage varies in quantity from sheet to sheet. In many cases, therefore, the finished size of a piezoelectric ceramic sheet may be larger or smaller than the design size, i.e., the nominal size, of the piezoelectric ceramic sheet. Thus, unevenness in individual piezoelectric ceramic sheets is inevitably produced in the finished size and the position of each active portion. For example, assuming that the positional difference between active portions of individual piezoelectric ceramic sheets is zero at the center of the lengths of the piezoelectric ceramic sheets, the positional difference increases as the distance of the active portions from the center increases. Therefore, in case of an actuator unit using a relatively large-sized piezoelectric ceramic sheet including a plurality of active portions, when the actuator unit is bonded to a passage unit with being positioned so that an active portion corresponds to a pressure chamber near the center of the length of the actuator unit, the positional difference between an active portion and a pressure chamber may be considerably large near either end of the actuator unit in the length of the actuator unit. As a result, uniform ink ejection performance of the ink jet head may not be obtained. To prevent this, only actuator units each having a finished size near the design size may be used as good products, thereby increasing uniformity in ink ejection performance. In this case, however, because the number of usable actuator units to the population parameter of interest decreases, the manufacture cost remarkably increases.
This problem is not limited to the case wherein an actuator unit includes a piezoelectric ceramic sheet in which active portions are formed by electrodes sandwiching the piezoelectric ceramic sheet. In case that an actuator unit including active portions may have relatively large dimensional error, the same problem may arise irrespective of the construction of the actuator unit.